The present invention relates to web conveying rollers, and more particularly to roller surface finish and a process for finishing the surface of a corona discharge treatment roller, so that dynamic air entrained between the roller surface and the web can be vented from the roller surface when the roller and web are in contact.
In many manufacturing operations a web is trained around a plurality of rollers as it is conveyed through a series of stations. Some of the rollers are drive rollers for advancing the web and other rollers are simple idler rollers. Typically the web conveyance system is designed to avoid relative movement between the surface of the rollers and the web in order to avoid scratching or otherwise damaging the web. This is especially important during the manufacture of sensitive materials, such as photographic films, paper and magnetic media where such relative movement can produce a surface defect in the final product. Thus it is desirable that the surface of the rollers be sufficiently smooth to avoid damage to the web by the rollers whether the rollers are drive rollers or idler rollers. At the same time, it is important that there be sufficient friction between the roller and the web to enable the corona discharge treatment (CDT) roller to be rotated by the web at the same velocity of the web when the web is in contact with the roller.
More particularly, in corona discharge treatment processes web is trained around a treatment roller as it is conveyed through the process. CDT rollers are simply smooth idler rollers and others work with a nip roller configuration. Existing CDT roller surfaces are made from silicone rubber, Hypalon(copyright) rubber, ceramic, epoxy quartz, glass or some metals. Typically the corona discharge treatment web conveyance system is designed to avoid relative movement between the surface of the treatment roller and the web in order to avoid scratching or other damage to the web. This is especially important during the manufacture of sensitive materials, such as photographic films and paper where such relative movement can produce a surface defect in the final product. Thus it is desirable that the surface of the rollers be sufficiently smooth to avoid damage to the web by the roller. At the same time, it is important that there be sufficient friction between the rollers and the web to enable the corona discharge treatment rollers to be rotated by the web at the same surface velocity of the web when the web is in contact with the roller.
As in other web transport systems air can become entrained between the roller and the surface of the web. Movement of the web and roller can force air into the entrance nip between the web and the surface of the roller, especially when the web is moving at high speeds. This forced air forms a boundary layer of air and can cause at least partial separation between the surface of the web and the surfaces of the rollers. When this occurs, there is a change in the ability of the corona discharge treatment process to effectively treat the face side of the web, and the web cannot efficiently rotate the idler rollers. As a result, relative movement can occur between the rollers and the web, causing quality defects in the web.
FIG. 1 illustrates a speed-traction performance curve for a web on a prior art smooth surfaced 8-inch diameter corona discharge treatment roller. As is apparent from the FIG. 1, the coefficient of friction between the web and the surface of a typical prior art type roller drops rapidly over the range of web speed up to 900 feet per minute. Thus, as higher roller speeds are required to meet demanding production schedules, existing roller surface traction is simply insufficient to convey the web without incurring enormous and expensive problems.
Smooth surfaced corona discharge treatment rollers are used when web speeds are low and the level of entrained air is low such that web/corona discharge treatment roller slippage is not a problem. At increased web speeds the use of a nip roller configuration is used to minimize entrained air and prevent web/corona discharge treatment roller slippage. However, nip rollers can not be used with certain film and paper webs that might be scratched.
There have been attempts to solve the problems caused by the boundary layer of air between a metal type roller and the web. One such prior art attempt is disclosed in U.S. Pat. No. 4,426,757. The web guide roller disclosed in such the ""757 patent has cavities on its outer surface which receive air carried with the moving web. More specifically, the cavities comprise a finely branched network of compression chambers that are arranged on the roller surface between plateau-like smoothly ground and polished areas which contact the web. Air in these chambers is compressed between the web and the roller. Air enters these chambers at the point where the web first contacts the roller, and the air is discharged from the chambers at the point where the web runs off the roller.
U.S. Pat. No. 3,405,855 discloses a plurality of grooves in the surface of a roller to control the air boundary layer. The grooves as disclosed in this patent provide passages for the discharge of the air. These grooves are specially formed in the surface of the roller in a predetermined periodic pattern, e.g., by a cutting operation. The grooves can leave thermal defects caused, for example, by the portion of a web in contact with the roller surface drying differently than the portion of the web over the grooves.
The problem with the use of venting patterns on corona discharge treatment rollers is the corona discharge treatment process can produce a latent image on the web from the corona discharge treatment roller venting pattern. After the web coating process the resulting patterned marks are easily observed by the human eye. These marks are clearly undesirable, especially in photographic products such as film or paper.
Therefore, a need persists for a web conveying roller that has a surface finish that eliminates the boundary air layer between the surface of the roller and the web, that is easy to manufacture and cost effective to produce.
It is, therefore, an object of the present invention to control dynamic air entrainment between a roller surface and a moving web at high speed.
Another object of the present invention is to vent air from between a roller surface and a moving web without requiring a specifically formed repeating pattern of grooves in the roller.
Yet another object of the invention is to provide a process for finishing the surface of the roller so that surface effectively vents air between the roller and the web supported on the surface.
Still another object of the invention is to provide an apparatus for conveying a web in a coating process on a corona discharge treatment roller that does not produce latent defects on the web.
Yet another object of the invention is to provide a corona discharge treatment roller with a surface finish having a non-periodic pattern formed by a shallow texture produced by the use of a laser engraving.
Accordingly for accomplishing these and other objects of the invention, there is provided, in one aspect of the invention, a web conveyance roller having a surface with generally down features, many of the down features overlapping to form interconnected channels comprising at least 50% of the surface area of the roller, the depth of the down features being greater than about 12 microns but less than about 125 microns, and a plurality of spatially separated plateaus between the channels comprising between at least 20% of the surface of the roller.
In another aspect of the invention, a process for finishing the surface of a roller of the type comprising materials selected from the group consisting of elastomers, ceramics, and epoxy quartz or a combination thereof, the roller being useful for conveying a web, the process comprising the steps of: providing a source of laser radiation; and, engraving the surface of the roller with radiation from the source of laser radiation to create on the surface a shallow texture with a pattern of (1) well rounded down features having a substantially uniform depth with many of the down features being interconnected and (2) generally curvatured plateaus.
In yet another aspect of the invention, an apparatus for conveying a web in a corona discharge coating process has at least one corona discharge treatment roller having a shallow textured surface with plateaus and down features as described above. Means is provided for applying a corona discharge to the web supported on the roller.
Accordingly, it is an important advantageous effect of the present invention that a roller for conveying a web in a corona discharge treatment process has a surface finish that eliminates the boundary layer air between the surface of the roller and the web. It is another advantageous effect of the present invention that the non-periodic pattern on the surface of the corona discharge treatment roller does not impart objectionable latent images on the web.